I believe release clauses are a great thing as it gives talent a way out for (presumably) a lesser wage-deal then and there. I believe any talent that intends to go through low or mid-table clubs should tell their agents to get a release clause that they both agree could be achievable with the progress they foresee.
I'm not sure how I feel about them for key-players at top clubs though. Especially as I'm a fan of a historically top club and currently looking to get 2nd place in the league after a fair few years below minimum expectations.
I'd like to think that there was loyalty enough in top football for a great player to give a club a few years to try and sort things out even if they had a easy out after a bad year, but I somehow doubt it considering loyalty is a two-way street that is rarely walker in today's game even when there isn't release clauses as a norm.
But yeah, if I was a professional footballer under 25 and not at a top club yet, I'd really want a release clause in my contract even if I had to sacrifice some wages to get it there. It would simplify a potential transfer a lot.
Of course, this is seeing from the outside looking in, so they might not see it as worth the reduced wages.